This invention relates to semiconductor devices, particularly to field-effect semiconductor devices as typified by the high electron mobility transistor (HEMT), and more particularly to such field-effect semiconductor devices that operate normally off. The invention also pertains to a method of fabricating such normally-off field-effect semiconductor devices.
HEMTs have been known which are made from semiconducting nitrides belonging to Groups III-V compounds. Japanese Unexamined Patent Publication No. 2005-158889 is hereby cited as dealing with this type of semiconductor devices which are pertinent to this invention. The prior art HEMT comprises an electron transit layer of undoped gallium nitride (GaN) grown on a silicon substrate via a buffer layer, and an electron supply layer of n-type aluminum gallium nitride (AlGaN) deposited on the electron transit layer. A source, a drain and a gate electrode are disposed in prescribed positions on the electron supply layer.
Made from the dissimilar semiconducting materials with unequal band gaps, the electron transit layer and electron supply layer provide a heterojunction therebetween. Spontaneous depolarization and piezoelectric depolarizations of this heterojunction provide in combination a channel of very low resistance, or of high electron mobility, for source-drain current flow under the control of a voltage applied to the gate. This channel is known to the specialists as a two-dimensional electron gas layer.
The HEMT of the general construction above was normally on; that is, there was a source-drain flow of electrons while no voltage was applied to the gate. The normally-on HEMT had to be turned off using a negative power supply for causing the gate to gain a negative potential. Use of such a negative power supply made the associated circuitry unnecessary complex and expensive. The conventional normally-on HEMT was rather inconvenient of use.
Attempts have been made to render the HEMT normally off by making the AlGaN electron supply layer thinner. A thinner electron supply layer weakens the electric field due to piezoelectric depolarization caused by the heterojunction between electron supply layer and electron transit layer, resulting in the diminution of electron concentration in the two-dimensional electron gas layer. The two-dimensional electron gas layer disappears just under the gate when a field due to the built-in potential between the electron supply layer and the Schottky gate acts upon the two-dimensional electron gas layer of reduced electron concentration. The HEMT can thus be held off between drain and source without application of a bias voltage to the gate.
The normally-off HEMT based upon this conventional scheme proved to possess its own drawback, however. As a result of the thin electron supply layer, the two-dimensional electron gas layer suffered an unnecessary drop in electron concentration at other than right below the gate, too. The result was an inconveniently high drain-source turn-on resistance.
A remedy to this inconvenience is found in Japanese Unexamined Patent Publication No. 2005-183733, which suggests to make the electron supply layer thinner only under the gate as by dry etching. The selective etching of the electron supply layer is objectionable because it is likely to lead to the impairment of the semiconductor crystal structures of both electron supply layer and electron transit layer. Selectively etching the electron supply layer without causing these inconveniences was indeed highly difficult, and the resulting HEMTs were not necessarily satisfactory in performance. For all these reasons, as far as the present applicant is aware, there seem to be no normally-off HEMTs that are available on the market today.
Japanese Unexamined Patent Publication No. 2006-100820 represents a different approach to the normally-off HEMT. It proposes to introduce a slant into part of the electron transit layer and electron supply layer. The source and drain electrodes are placed on the electron supply layer on a level with the top and bottom, respectively, of slant, and the gate electrode is positioned on the slant itself via an insulating seat. This proposed construction is not necessarily reliable for stable HEMT operation in normally-off mode.
There have been consistent demands from electronics and allied industries for not only normally-off two-dimensional-electron-gas HEMTs but normally-off two-dimensional-hole-gas HEMTs and other such field-effect transistor devices too.